Influence of operating conditions on the optical optimization of solar selective absorber coatings

Abstract

In concentrated solar power (CSP), selective solar absorber coatings increase absorbed solar flux (high solar absorptance AS) while reducing radiative losses (low thermal emittance EBB). Coating developers usually maximize the selective ratio As/EBB (highest As, lowest EBB). The solar-to-heat conversion (heliothermal) efficiency Rh of a solar absorber is however a more relevant optimization target. It is deduced from As and EBB, but is more sensitive to As. It also strongly depends on operating conditions, solar concentration ratio C and absorber temperature TA, radically varying between CSP technologies. This allows different optimal designs and leads to more efficient selective coatings, adapted to their specific application. In this paper, the influence of operating conditions on the optimal selective coating design is investigated. A popular selective coating design, W/W–Al2O3/Al2O3, was chosen as illustrative example. Coating structure and composition, i.e., cermet composition and layer thicknesses, were optimized to maximize Rh for a wide range of operating conditions: concentration ratios from 10 to 300, absorber temperatures from 50 to 550 °C. Optimal coating characteristics are illustrated on 3D maps over the entire (C, TA) range, showing the existence of three operating regions. Each region gives an optimization priority on either As, EBB or a combination of both, leading to specific optimized coating characteristics for each region, which should be considered when designing selective coatings for given operating conditions. Based on one example, this study provides a better understanding of the optimal design and use of selective coatings in the different operating conditions of CSP technologies.